You can build a DIY reactor for your reef tank for $30-80 in materials, depending on the type, and achieve performance comparable to commercial units costing $100-400. The most common DIY reef reactors are calcium reactors (using CO2 to dissolve calcium carbonate media), media reactors for GFO or activated carbon, and biopellet reactors for nitrate and phosphate control. Each has a different internal design, but they all share the same basic construction: a sealed chamber, an inlet and outlet, and a recirculation or flow-through system.
This guide covers the design principles for each reactor type, step-by-step build instructions for a calcium reactor (the most requested DIY reef reactor), where to source materials, and the key differences between good DIY designs and poorly built ones that leak or underperform.
The Role of Reactors in a Reef Tank
Reef tanks have higher water chemistry demands than fish-only systems. SPS and LPS corals consume calcium (target 400-450 ppm) and alkalinity (target 8-12 dKH) continuously to build their calcium carbonate skeletons. As coral growth increases, manual two-part dosing becomes expensive and a calcium reactor becomes more cost-effective.
Reactors solve different problems:
Calcium reactor: Dissolves calcium carbonate (aragonite or calcite) media using CO2-acidified water, releasing calcium and alkalinity into the tank. One 5-lb bag of ARM Coarse Aragonite media can maintain a mature 150-gallon reef for several months, at a fraction of the cost of two-part additives.
Media/GFO reactor: Runs granular ferric oxide (GFO) in a tumbling media bed to remove phosphate. Or activated carbon for DOC removal. These are simple flow-through reactors with screens to contain media.
Biopellet reactor: Uses solid organic carbon pellets in a tumbling reactor for combined nitrate and phosphate reduction through bacterial uptake.
DIY Calcium Reactor: The Build That Pays Off Most
A calcium reactor is where DIY saves the most money relative to commercial alternatives. Commercial calcium reactors like the GEO 618 or Korallin C-1502 run $250-500. A DIY version using 3-inch acrylic tube costs around $50-80 and handles equivalent tank volume.
Materials for a Dual-Chamber Calcium Reactor
A dual-chamber design is the standard for larger reefs because it provides a second contact stage that raises the pH of effluent before it returns to the tank. This prevents pH crashes from low-pH effluent.
Chamber 1 (primary): 3-inch clear acrylic tube, 16-18 inches long. This is where CO2 is injected and media dissolves.
Chamber 2 (secondary): 3-inch clear acrylic tube, 6-8 inches long. Effluent from chamber 1 passes through additional media here, raising its pH by consuming the remaining dissolved CO2.
End caps: 4 acrylic discs cut to 3.25 inches diameter, bonded with Weld-On #4.
Bulkhead fittings: 4 bulkheads total, 3/8-inch size. One inlet and one outlet per chamber.
Recirculation pump: A small mag-drive pump (Rio Nano 50 or Sicce Syncra 0.5, both around $15-25) to circulate water inside chamber 1 past the media continuously, separate from the main feed flow.
CO2 injection port: A small barbed fitting on chamber 1 for the CO2 line.
Silicone sealant and Weld-On acrylic cement
Media: ARM Coarse Aragonite media, roughly $15-20 for a 10-lb bag (more than enough to fill both chambers).
Total materials: approximately $60-90 depending on pump choice and acrylic tube source.
Step-by-Step Build
Step 1: Cut acrylic tube sections. Use a fine-toothed table saw or circular saw with an acrylic-rated blade. Cut your primary chamber to 16-18 inches and secondary to 6-8 inches. Sand cut ends with 400-grit sandpaper until smooth.
Step 2: Drill end caps. Cut four 3.25-inch discs from 0.25-inch acrylic sheet. Drill bulkhead holes sized for your fittings (usually 7/8-inch hole for a 3/8-inch bulkhead). Drill the CO2 port hole in one end cap (bottom of chamber 1). Drill the recirculation pump intake and outlet holes in the bottom and side end caps of chamber 1.
Step 3: Bond end caps. Apply Weld-On #4 around the inside joint between end cap and tube. Work quickly, Weld-On sets in about 60 seconds. Hold for 30 seconds. Full cure in 24 hours.
Step 4: Install bulkheads and fittings. Thread bulkheads through drilled holes with provided gaskets. Hand-tighten plus a quarter turn with channel-locks. Apply silicone around the outside base of each fitting. Install CO2 injection fitting with silicone.
Step 5: Pressure test. Fill with water, let sit 24 hours. Check all joints. Any seepage means more silicone and another 24-hour cure.
Step 6: Install recirculation pump. The pump mounts inside the primary chamber (most Rio Nano size pumps fit through the end cap before bonding, then the wiring exits through a small cable seal) or externally with external intake/outlet fittings. Internal mounting is simpler. External gives easier maintenance access.
Step 7: Load media and connect.
For more calcium reactor options and comparisons, see our Best Aquarium Equipment guide.
Connecting and Running the Calcium Reactor
The calcium reactor feeds from your return pump output (or a small dedicated pump from the sump), passing a low flow rate through the reactor. Typical feed flow is 20-60 mL per minute, controlled by a needle valve or peristaltic pump.
CO2 from your regulator enters chamber 1. Bubble count inside the reactor (not in your tank) should be 1-3 bubbles per second for most systems. The recirculation pump cycles water inside chamber 1 continuously, keeping media in contact with CO2-acidified water.
Effluent drips from chamber 2 back to the sump at the same rate as the feed flow. Effluent pH should be 6.4-6.8. You measure this with a probe in the effluent line. If pH is above 6.8, increase CO2 bubble rate or decrease feed flow. If below 6.3, you're dissolving media aggressively but may affect tank pH. Most reactors run chamber 1 effluent at 6.2-6.5 and chamber 2 raises this to 6.8-7.0 before it hits the sump.
Tuning Calcium and Alkalinity Output
The two variables you're adjusting are: CO2 bubble rate (determines how acidic the chamber water gets and how much media dissolves) and feed flow rate (determines how much effluent per hour enters the sump).
Start by testing your tank's baseline calcium and alkalinity consumption. If a 150-gallon SPS reef with 50 medium-sized colonies consumes about 1 dKH per day, you can calculate the daily alkalinity dose needed and work backward to effluent flow rate and target chemistry.
This takes 2-4 weeks to dial in. Test calcium and alkalinity every 2-3 days during the initial period. Make one change at a time.
DIY Media Reactor Build (Simpler Alternative)
If you just need a GFO or carbon reactor, the build is much simpler than the calcium reactor:
4-inch PVC pipe, 12 inches long, with two end caps drilled for barbed hose fittings. Two foam discs cut to fit inside as media screens. Load with 100-200mL of BRS ROX 0.8 activated carbon or BRS GFO media. Connect to a small pump at 100-200 LPH.
Cost: about $15-25. This handles carbon and phosphate reduction for tanks up to 200 gallons with a single reactor body.
The Top Aquarium Equipment guide covers commercial reactor options for comparison.
Common DIY Reactor Mistakes
Using standard vinyl tubing for CO2 lines. CO2 permeates standard vinyl. Use CO2-rated polyurethane or PVC tubing with CO2 diffusers. Small CO2 losses through the walls add up over weeks.
Skipping the pressure test. A leak that appears after hours of operation is much more problematic than one caught on the bench. Always pressure-test before final installation.
Reactor media packed too tightly. For media reactors with tumbling media, overfilling prevents movement. For calcium reactors, packing is fine (media is static) but make sure the recirculation pump can move water through the media bed.
Not accounting for pH impact. A calcium reactor running at 6.3 effluent pH adds 20-50 mL/min of low-pH water to your sump. In a lightly buffered system, this can pull tank pH down noticeably in the afternoon when photosynthesis stops. The dual-chamber design mitigates this.
FAQ
What's the best media for a DIY calcium reactor? ARM Coarse Aragonite is the hobbyist standard. It dissolves reliably, provides both calcium and alkalinity in the correct ratio (close to Redfield ratio for calcium carbonate), and is cheap at $15-20 for 10 lbs. Koralith media and Caribsea Reactor Media are alternatives. Avoid using crushed coral: it's less pure calcium carbonate and includes phosphate-releasing organic material.
Can I build a calcium reactor without dual chambers? Yes. Single-chamber reactors work fine for smaller tanks and lighter coral loads. The trade-off is that effluent pH is lower (more acidic) when it hits the sump, which can cause visible pH drops in the tank. If your display tank has strong gas exchange and consistent pH, a single chamber is acceptable. For SPS-heavy tanks or those with stable pH requirements, dual-chamber is the better design.
How do I know the right CO2 bubble rate for my reactor? There's no universal number because it depends on your tank's alkalinity consumption, feed flow rate, and media type. Start at 1-2 bubbles per second inside the reactor and test your alkalinity every 2 days. If alkalinity is rising above your target, reduce CO2 or increase feed flow. If alkalinity is falling below target, increase CO2 or decrease feed flow. Expect 3-6 weeks to fully stabilize.
Do I need a CO2 controller or is a standard regulator enough? A standard dual-stage regulator with a needle valve and bubble counter is sufficient for most DIY calcium reactors. CO2 controllers (which monitor tank pH and adjust CO2 automatically) are a convenience upgrade but not required. The main thing the needle valve and bubble counter can't do is respond automatically to changes in coral growth or bioload. You tune manually every few weeks instead.
Conclusion
The DIY calcium reactor build using 3-inch acrylic tube, four bulkheads, and a small recirculation pump produces real results on a reef tank for under $100 in materials. The payoff is most obvious on larger, heavily-calcifying reefs where two-part dosing costs add up fast. Get the pressure test and CO2 connections right before the first fill, and spend the first month in calibration mode rather than expecting instant stability. A calcium reactor that's dialed in is almost set-and-forget maintenance compared to daily dosing routines.